A vehicle seat is typically provided with a pair of lower rails supported on a floor of a vehicle body so as to extend in the fore and aft direction in parallel to each other, a pair of upper rails engaged by the respective lower rails for slide motion in the lengthwise direction thereof, a seat cushion attached to the upper rails, a seat back connected to a rear end part of the seat cushion, a lock mechanism for normally securing the upper rails relative to the lower rails at desired positions of the seat cushion, and a lock release lever for manually and selectively releasing the locked state of the lock mechanism. See JP2005-225386A and JP2008-265695A, for instance.
As the lower rails support the seat, it is important that the lower rails are able to withstand the loading caused by a vehicle crash in order to ensure the safety of the vehicle occupant at the time of the vehicle crash. However, using thicker and more massive material for the lower rails leads to an excessive increase in the weight thereof, and this is detrimental to the fuel economy of the vehicle.